A keyboard is a primary means of communication between persons and machines such as computers. For example, a keyboard can be used to provide an input, for example, to a computer or to the logic unit of, for example, a process controller. Such keyboards can include, for example, control keyboards suitable for installation in a petroleum, petrochemical, or chemical process plant. However, to accomplish this interfacing function between persons and machines, it is necessary that keys representing a particular instruction be translated to the data format of the computer, for example, to binary form. Generally, this involves translating, for example, alphanumeric significations assigned to keys on a keyboard and having meaning for a human operator to binary form for processing by a digital computer. For example, on a keyboard suitable for installation in a petroleum, petrochemical, or chemical process plant, such keyboards can include keys to "enter set points", "enter controller tuning constants", "enter alarm limits", "transfer control loops", and the like.
Such keyboards can be utilized in environments characterized as hazardous because of the presence of volatiles or inflammables for example, in petroleum plants, petrochemical plants, and other plants. In such circumstances, it can be desirable that the keyboard unit be located at a considerable distance from the computer and have design features furthering safety. To facilitate transmission of binary data from a keyboard to a computer processing unit, it is desirable to convert the binary word into serial form. To further safety, it is desirable that the keyboard encoder be so far as possible, intrinsically safe. Further since such a keyboard encoder is located at a considerable distance from the computer processing unit it is desirable that the keyboard also be capable of decoding binary form data words from the processing unit into signs or signals having signification for the human operator so that two way communication between the keyboard operator and the processing unit is achieved.
To further safety, as indicated, it is desirable that intrinsically safe components be utilized. Hall effect type switches have been developed which do not rely on a metallic contact closure to connect to signal lines. Such switches are capable of supplying two arc-free bounceless outputs without generating radio frequency interference. This type of switch acts as a current sink for two signal lines. The fact that no physical contact is required by the Hall effect type switches enhances the life and the reliability of the keyboard itself by virtue of the fact that there is no contact erosion or bounce. It is thus desirable to use Hall effect type switches in keyboard systems. However, it has been difficult in the past to interface a keyboard using Hall effect type switches to a computer in serial binary format.
In addition, where a keyboard is utilized, for example, to control the logic processor of, for example, a process controller, it is highly desirable so far as possible that only valid data in a valid format be encoded and transmitted to the processor. By the term "valid data" is meant that the data encoded is the instruction intended by the human operator. By the term "valid format" is meant that the data is in proper form for processing by, for example, a logic processor whether or not the data is valid data.
It can readily be appreciated that a frequent source of invalid data is the simultaneous or near simultaneous depressing of two or more keyboard switches on a switchboard. Such multiple key actuation can, in some keyboard encoders, result in transmission of a composite binary data word which is not representative of any of the actuated keys.
Accordingly, an object of the invention is apparatus and method for encoding the output of a keyboard into serial binary form. A further object is such an apparatus and method capable of transmitting and receiving serial binary data to an external device. A yet further object is such an apparatus and method functional for decoding the thus received binary serial data. Another object is apparatus and method for reducing errors caused by the simultaneous or near simultaneous depression of more than one key. Yet another object is such an encoder decoder which can operate from a single power supply and which can be operated at a considerable distance from the external device with which it is in communication. Yet another object is such an apparatus and method which is well adapted for its intended use. Other objects and advantages of the invention can appear to one skilled in the art from a consideration of the following description and the drawings.